Many semiconductor devices require multiple voltage sources for operation. In particular, nonvolatile memories such as Flash memory and Electrically Erasable Programmable Read Only Memory (EEPROM) generally require an operating voltage and other relatively high voltages to perform write and erase operations. Other voltage levels may be required on memories and many other semiconductor devices for supplying sense amplifiers, analog devices, fuses, anti-fuses, input/output devices, analog-digital converters, digital-to-analog converters and other similar devices.
Conventionally, most of these voltage sources have been supplied externally from the semiconductor device and brought into the device through bonding pads. With external voltage sources, the voltage level can be controlled precisely and used in multiple devices within a system. However, there is a limit to the number of voltages a system can support and the number of bonding pads and pins that a semiconductor device can dedicate to voltage sources.
Recently, internal voltage generators have been used to generate the increasing number of different voltage levels that may be required on modern semiconductor devices. Some of these voltage generators have been programmable so the voltage generator can produce a variety of different voltage levels based on the programmed value. However, many of these programmable voltage generators use two or more signals that must be at the same potential. A difference in potential on these signals can lead to inaccuracies in the resulting voltage level.
Furthermore, many of these programmable voltage generators are implemented with a large number of resistors for generating a variety of current levels, which are then used to generate the various programmable voltage levels. However, resistors can be difficult to fabricate precisely on semiconductor devices. Even if the resistors are fabricated with enough precision, they may also require significant real estate on the semiconductor device.
FIG. 1 is a simplified circuit diagram of a conventional programmable voltage regulator 10. The voltage regulator 10 includes a comparator 12, a pump control 14, a charge pump 16, a feedback resistor RF connected between the output of the charge pump 6 and node N1, and a first load resistor RL1 connected between node N1 and ground.
The comparator 12 compares node N1 to a voltage reference VREF to generate a signal for the pump control 14. The pump control 14 uses this signal to control the charge pump 16, which generates a pumped voltage Vpump to drive the feedback resistor RF.
Node N1 is also coupled to a digital-to-analog converter. For each bit of the digital input (A, B, C), the digital-to-analog converter includes a pair of n-channel transistors coupled to a resistor of a conventional R-2R resistor ladder, which is connected to a second load resistor RL2. One n-channel transistor from the pair of transistors is connected to node N1 and the other transistor from the pair of transistors is connected to node N2. Node N2 is connected to a voltage reference generator 20.
In operation, the nodes N1 and N2 are configured to be at the same potential such that they appear as virtual nodes of each other. As a result, each pair of n-channel transistors, when enabled, conducts the same amount of current because one or the other of the pair of transistors is enabled. As an example, if A is high, A# will be low and node N1 will conduct through the transistor coupled to A. Similarly, if B is low and B# is high, node N2 will conduct through the transistor coupled to B#. In this fashion, a constant current is supplied to the R-2R resistor ladder regardless of the digital input value. However, some of the current will flow through N1 and some of the current will flow through N2 based on the value on the digital input (A, B, C). Thus, the value on the digital input selects how much current flows on N1 to affect the feedback mechanism of the comparator 12, pump control 14, and charge pump 16, resulting in an output voltage Vpump related to the digital input.
However, if the voltage on N2 drifts, relative to the VREF signal at the comparator 12, the voltage on N1 may drift relative to N2 resulting in inaccuracies in the digital-to-analog converter. In addition, the R-2R resistor ladder includes many resistors. These resistors may be difficult to fabricate accurately and may occupy substantial space on a semiconductor device.
There is a need for apparatuses, systems, and methods to generate and regulate voltage levels on a semiconductor device. There is also a need to make these voltage levels easily programmable with reduced vulnerability to noise levels and with devices that are easily fabricated.